Malonaldehyde is widely produced in mammalian tissue as a fragmentation product of prostaglandin endoperoxides and an end product of lipid peroxidation. Although it undoubtedly has universal occurence, very little is known about the basic biochemistry of malonaldehyde and its role in physiological processes. This information has taken on added significance with the recent findings that malonaldehyde is mutagenic and carcinogenic. The principal reason for our ignorance of the biochemistry of malonaldehyde has been the lack of radioactively labeled material for in vivo and in vitro studies. We have synthesized malonaldehyde uniformly labeled with 14C. In the course of this work we discovered that the common procedure for the preparation of malonaldehyde (i.e. hydrolysis of tetraalkoxypropanes) produced appreciable concentrations of side products. Since this is the method which was used for the estimation of the mutagenicity and carcinogenicity studies the possibility exists that malonaldehyde is not mutagenic and carcinogenic but that one of the side products of the preparation was responsible for this behavior. We, therefore, propose to determine unequivocally whether pure malonaldehyde is a mutagen and carcinogen. We plan to use our radiolabeled material to elucidate the pathways for the metabolism of malonaldehyde. Since covalent binding to nucleic acids and proteins is often suggested as an important step in the development of neoplastic states, we describe experiments designed to estimate the relative rates of macromolecular binding and metabolism in vivo and in vitro. These experiments and others will serve as a basis for our long term study of the physiological role of malonaldehyde and its involvement in the evolution of pathological states.